Processes and reactors for the batchwise (discontinuous) production of condensation polymers, in particular polyesters, in the melt phase, and the granulation of the product melts effected subsequent to the polycondensation are known. The particular importance of the batch process consists in its flexibility with respect to changed product formulations and in a low-loss change of the product formulation. Existing batch plants are mostly used for the synthesis of polyethylene terephthalate (PET) and copolyesters on the basis of PET. Subsequent to a first process stage for producing a prepolymer (as esterification product in the terephthalic acid process or as transesterification product in the dimethyl terephthalate process) a stirred autoclave is used for the polycondensation of the prepolymer under a vacuum. The stirred autoclaves of conventional plants are upright reactors with vertical stirrer shaft and special helical stirrers adapted to the bottom of the tank (hemispherical or conical).
In a PET batch process, for example, the disadvantages of these known stirred autoclaves are:    a) batch sizes up to a maximum of 4 to 6 t;    b) a geometrically limited, specific surface area of about 1.3 m2/t in the reactor;    c) a product conveyance limited in vertical direction (stirrer movement predominantly in horizontal peripheral direction);    d) a restricted intensity of the mass exchange with the practical consequence of longer reaction times, limited plant capacities and viscosities (IV≦0.66 dl/g), forced final operating vacua (not more than 0.4 mbar abs.), an economic mode of operation only with an increased input of mechanical energy and elevated final polymer temperatures (≧290° C.), losses of capacity and viscosity restrictions at reduced temperatures or operating vacua, final polymer temperatures increasing with increasing batch or plant size;    e) increased viscosity fluctuations, e.g. ΔIV≦0.03 dl/g with IV=0.65 dl/g, in the granules due to the thermal decrease in viscosity of the melt during granulation as well as additional deviations of ΔIVR≦0.01 dl/g in the batch sequence;    f) polymer losses of about 0.65 wt-% during the regular rinsing of the discharge system prior to granulation;    g) increased acetaldehyde content and reduced color quality of the PET polymers.
To accelerate the reaction in the conventional autoclave, a surface increase up to 1.5 times the amount by inclining the container axis up to 48° from the vertical has been proposed (EP 0 753 344 A2). However, objections against this measure result from a complicated, expensive mechanism of the stirrer and the uncertainty as regards the control of transverse forces and unbalanced masses.
Better possibilities for increasing the specific product surface (by a factor of about 15 to 30) and for increasing the product viscosities can be realized in a lying ring-disc reactor with entraining surfaces of the stirrer rotating substantially in vertical paths and with free-falling films (mists) inside the ring discs (U.S. Pat. Nos. 3,499,873 and 3,617,225). The main problem of cylindrical ring-disc reactors tested in batch operation was an insufficient drainage behavior of the apparatus. This is even more true for ring-disc reactors with a frustoconical shape and with the product outlet in the part of the reactor with the smallest diameter. Non-walkable reactors also have assembly problems during the installation of baffle plates. The reactors including ring discs carried by a cage, which are described in the U.S. Pat. No. 4,540,774, are only designed for the continuous operation.
The polycondensation tank tapered at both ends, which is described in D.D. patent 52942, exhibits a better drainage behavior. The scoop-like stirrer elements, however, limit the formation of free product surfaces and are not suited for higher viscosities.
If one sticks to the concept of a stirring disc autoclave with individual stirring disc elements separated by scraper elements (U.S. Pat. Nos. 3,346,242 and 3,880,407) in favor of high final polymer viscosities (e.g. IV ≦0.82 dl/g in the case of PET), there are further problems as regards the reduced intensity of the mass exchange at the container wall in the case of low viscosities and the restricted heat transfer. Under technical production conditions with a relative filling level h/D=0.4 and usual rotational speeds ≦8 rpm, PET has the disadvantage of                the necessity of preheating the condensate to about 280° C.,        a restricted polycondensation performance with a maximum of 7 batches per day, and        a reduced polymer color and increased viscosity fluctuations.        
It is the object of the present invention to minimize the aforementioned disadvantages, and in particular substantially improve the existing potential of batch plants to form multipurpose purpose plants by a more efficient polycondensation technique and a more uniform quality of the granules.